Porous PTFE space-charge electrets for piezoelectric applications

Porous polytetrafluoroethylene (PTFE) films were positively or negatively c orona charged at room or elevated temperatures. Their charge storage behavi or was investigated by means of isothermal surface potential measurements i n direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge s tability for both polarities. Direct piezoelectricity was studied on quadru ple, double, and single layer samples by means of quasi-static measurements . For the determination of indirect piezoelectricity, frequency-dependent a coustical-transducer experiments were carried out. Both applications-releva nt measurements yielded piezoelectric d(33) coefficients of up to approxima tely 600 pC/N or 600 pm/V. These values are more than one order of magnitud e higher than in conventional piezoelectric polymers such as polyvinylidene fluoride (PVDF) and almost comparable to the highest known values of inorga nic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropoly mer spacecharge electrets exhibit an outstanding potential for various devi ce applications that are very briefly discussed.